1. Field of the Invention
The present invention relates generally to methods and devices for providing laser light for optical systems and more particularly relates to superfluorescent fiber sources for providing laser light with enhanced mean wavelength stability.
2. Description of the Related Art
Er-doped superfluorescent fiber sources (SFSs) have been studied extensively for their application in fiber optic gyroscopes (FOGs). SFSs exhibit a unique combination of high efficiency, high spatial coherence, broad spectral emission, and excellent long-term stability of the mean wavelength. See, e.g., D. G. Falquier, “Erbium doped superfluorescent fiber sources for the fiber optic gyroscope,” Ph. D. dissertation, December 2000, Applied Physics Department, Stanford University, Stanford, Calif.; D. C. Hall et al., “High-stability Er3+-doped superfluorescent fiber sources,” J Lightwave Tech., Vol. 13, No. 7, pp. 1452-1460, July 1995; and P. F. Wysocki et al., “Characteristics of erbium-doped superfluorescent fiber sources for interferometric sensor applications,” J. Lightwave Tech., Vol. 12, No. 3, pp. 550-567, March 1994, each of which is incorporated in its entirety by reference herein.
Long term stability of the mean wavelength of the SFS is desirable because the scale factor of a FOG scales with the mean wavelength of the source. Therefore, accurate knowledge of the scale factor, and thus of the mean wavelength, is particularly useful for accurate measurements of the absolute rotation rate from the FOG. D. C. Hall et al. (cited above) have reported a mean wavelength stability for an Er-doped SFS of the order of 8 parts per million (ppm). This mean wavelength stability is adequate for low- to medium-accuracy FOGs. However, high-accuracy inertial navigation FOGs utilize a higher stability of the mean wavelength over many hours.
The prior art does not report an SFS with a sufficient mean wavelength stability for high accuracy applications. One reason for this is that it is difficult to stabilize the various parameters upon which the mean wavelength of an SFS depends (e.g., the wavelength, power, and polarization of the pump, the temperature and birefringence of the fiber, and the optical feedback returning from the FOG). Previous efforts have provided detailed studies of the contributions of these individual parameters to the mean wavelength and have reported various methods of effectively reducing these contributions and/or the variability of these contributions. Besides the exemplary reports of such previous efforts of D. C. Hall et al. and of P. F. Wysocki et al., both cited above, other exemplary reports include T. Gaiffe et al., “Wavelength stabilization of an erbium-doped-fiber source with a fiber Bragg grating for high-accuracy FOG,” Proc. SPIE, Vol. 2837, pp. 375-380, 1996; H. J. Patrick et al., “Erbium-doped superfluorescent fibre source with long period fibre grating wavelength stabilisation,” Electron. Lett., Vol. 33, No. 24, pp. 2061-2063, 1997; M. J. F. Digonnet, “Broadband fiber sources,” in Rare-Earth-Doped Fiber Lasers and Amplifiers, pp. 313-340, 2001, 2nd Edition, M. J. F. Digonnet, Ed., Marcel Dekker, Inc., New York; P. Wysocki et al., “Wavelength Stability of a High-Output, Broadband, Er-Doped Superfluorescent Fiber Source Pumped near 980 nm,” Opt. Lett, Vol. 16, No. 12, pp. 961-963, June 1991; and P. Z. Zatta et al., “Ultra-high-stability two-stage superfluorescent fibre sources for fibre optic gyroscopes,” Electron. Lett., Vol. 38, No. 9, pp. 406-408, April 2002, each of which is incorporated in its entirety by reference herein.
The dependence of the mean wavelength on the pump wavelength has been previously reduced by selecting the pump wavelength appropriately and by stabilizing the laser diode temperature and current. The pump power dependence of the mean wavelength has been previously reduced by proper selection of the pump power and fiber length. Stabilizing the laser diode temperature and stabilizing the current have also been used to reduce the pump power dependence of the mean wavelength. The effects of optical feedback on the mean wavelength can be reduced, and even cancelled, by proper design of the SFS configuration and by optically isolating the SFS and the gyro coil. These contributions to the mean wavelength have thus been reduced to a few ppm level or less, but further stabilization is still desirable.